/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
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 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package com.jerry.walking.utils;

import java.util.Collection;

/**
 * A collection designed for holding elements prior to processing. Besides basic
 * {@link Collection Collection} operations, queues provide additional
 * insertion, extraction, and inspection operations. Each of these methods
 * exists in two forms: one throws an exception if the operation fails, the
 * other returns a special value (either <tt>null</tt> or <tt>false</tt>,
 * depending on the operation). The latter form of the insert operation is
 * designed specifically for use with capacity-restricted <tt>Queue</tt>
 * implementations; in most implementations, insert operations cannot fail.
 * <p/>
 * <p/>
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 * <tr>
 * <td></td>
 * <td ALIGN=CENTER><em>Throws exception</em></td>
 * <td ALIGN=CENTER><em>Returns special value</em></td>
 * </tr>
 * <tr>
 * <td><b>Insert</b></td>
 * <td>{@link #add add(e)}</td>
 * <td>{@link #offer offer(e)}</td>
 * </tr>
 * <tr>
 * <td><b>Remove</b></td>
 * <td>{@link #remove remove()}</td>
 * <td>{@link #poll poll()}</td>
 * </tr>
 * <tr>
 * <td><b>Examine</b></td>
 * <td>{@link #element element()}</td>
 * <td>{@link #peek peek()}</td>
 * </tr>
 * </table>
 * <p/>
 * <p/>
 * Queues typically, but do not necessarily, order elements in a FIFO
 * (first-in-first-out) manner. Among the exceptions are priority queues, which
 * order elements according to a supplied comparator, or the elements' natural
 * ordering, and LIFO queues (or stacks) which order the elements LIFO
 * (last-in-first-out). Whatever the ordering used, the <em>head</em> of the
 * queue is that element which would be removed by a call to {@link #remove() }
 * or {@link #poll()}. In a FIFO queue, all new elements are inserted at the
 * <em> tail</em> of the queue. Other kinds of queues may use different
 * placement rules. Every <tt>Queue</tt> implementation must specify its
 * ordering properties.
 * <p/>
 * <p/>
 * The {@link #offer offer} method inserts an element if possible, otherwise
 * returning <tt>false</tt>. This differs from the
 * {@link Collection#add Collection.add} method, which can fail to add
 * an element only by throwing an unchecked exception. The <tt>offer</tt> method
 * is designed for use when failure is a normal, rather than exceptional
 * occurrence, for example, in fixed-capacity (or &quot;bounded&quot;) queues.
 * <p/>
 * <p/>
 * The {@link #remove()} and {@link #poll()} methods remove and return the head
 * of the queue. Exactly which element is removed from the queue is a function
 * of the queue's ordering policy, which differs from implementation to
 * implementation. The <tt>remove()</tt> and <tt>poll()</tt> methods differ only
 * in their behavior when the queue is empty: the <tt>remove()</tt> method
 * throws an exception, while the <tt>poll()</tt> method returns <tt>null</tt>.
 * <p/>
 * <p/>
 * The {@link #element()} and {@link #peek()} methods return, but do not remove,
 * the head of the queue.
 * <p/>
 * <p/>
 * The <tt>Queue</tt> interface does not define the <i>blocking queue
 * methods</i>, which are common in concurrent programming. These methods, which
 * wait for elements to appear or for space to become available, are defined in
 * the {@link java.util.concurrent.BlockingQueue} interface, which extends this
 * interface.
 * <p/>
 * <p/>
 * <tt>Queue</tt> implementations generally do not allow insertion of
 * <tt>null</tt> elements, although some implementations, such as
 * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>. Even in the
 * implementations that permit it, <tt>null</tt> should not be inserted into a
 * <tt>Queue</tt>, as <tt>null</tt> is also used as a special return value by
 * the <tt>poll</tt> method to indicate that the queue contains no elements.
 * <p/>
 * <p/>
 * <tt>Queue</tt> implementations generally do not define element-based versions
 * of methods <tt>equals</tt> and <tt>hashCode</tt> but instead inherit the
 * identity based versions from class <tt>Object</tt>, because element-based
 * equality is not always well-defined for queues with the same elements but
 * different ordering properties.
 * <p/>
 * <p/>
 * <p/>
 * This interface is a member of the <a href="{@docRoot}
 * /../technotes/guides/collections/index.html"> Java Collections Framework</a>.
 *
 * @param <E> the type of elements held in this collection
 * @author Doug Lea
 * @see Collection
 * @see LinkedList
 * @see PriorityQueue
 * @see java.util.concurrent.LinkedBlockingQueue
 * @see java.util.concurrent.BlockingQueue
 * @see java.util.concurrent.ArrayBlockingQueue
 * @see java.util.concurrent.LinkedBlockingQueue
 * @see java.util.concurrent.PriorityBlockingQueue
 * @since 1.5
 */
public interface Queue<E> extends Collection<E>
{
    /**
     * Inserts the specified element into this queue if it is possible to do so
     * immediately without violating capacity restrictions, returning
     * <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>
     * if no space is currently available.
     *
     * @param e the element to add
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     * @throws IllegalStateException    if the element cannot be added at this time due to capacity
     *                                  restrictions
     * @throws ClassCastException       if the class of the specified element prevents it from being
     *                                  added to this queue
     * @throws NullPointerException     if the specified element is null and this queue does not
     *                                  permit null elements
     * @throws IllegalArgumentException if some property of this element prevents it from being added
     *                                  to this queue
     */
    boolean add(E e);

    /**
     * Inserts the specified element into this queue if it is possible to do so
     * immediately without violating capacity restrictions. When using a
     * capacity-restricted queue, this method is generally preferable to
     * {@link #add}, which can fail to insert an element only by throwing an
     * exception.
     *
     * @param e the element to add
     * @return <tt>true</tt> if the element was added to this queue, else
     * <tt>false</tt>
     * @throws ClassCastException       if the class of the specified element prevents it from being
     *                                  added to this queue
     * @throws NullPointerException     if the specified element is null and this queue does not
     *                                  permit null elements
     * @throws IllegalArgumentException if some property of this element prevents it from being added
     *                                  to this queue
     */
    boolean offer(E e);

    /**
     * Retrieves and removes the head of this queue. This method differs from
     * {@link #poll poll} only in that it throws an exception if this queue is
     * empty.
     *
     * @return the head of this queue
     * @throws NoSuchElementException if this queue is empty
     */
    E remove();

    /**
     * Retrieves and removes the head of this queue, or returns <tt>null</tt> if
     * this queue is empty.
     *
     * @return the head of this queue, or <tt>null</tt> if this queue is empty
     */
    E poll();

    /**
     * Retrieves, but does not remove, the head of this queue. This method
     * differs from {@link #peek peek} only in that it throws an exception if
     * this queue is empty.
     *
     * @return the head of this queue
     * @throws NoSuchElementException if this queue is empty
     */
    E element();

    /**
     * Retrieves, but does not remove, the head of this queue, or returns
     * <tt>null</tt> if this queue is empty.
     *
     * @return the head of this queue, or <tt>null</tt> if this queue is empty
     */
    E peek();
}
